In general, in order to remove a spot-welded portion to be used for a body panel or the like of a vehicle, as shown in FIG. 28, a so-called candle-type drill, in which a central portion of a point of a drill is formed into an angle-shaped and projected portion and an outer peripheral portion is formed into a substantially plane shape, is used.
This is because in the case where a drill with a point angle of 118.degree. to be used generally is used, the point of the drill pierces into a spot-welded portion, and a hole is made not only in a panel which is desired to be replaced but also in a panel which is desired to remain. On the contrary, when the candle-type drill is used, only a projection of the point of the drill cuts into the welded portion and thus a cutting center is achieved, and the welded portion is cut by a cutting edge on the outer side of the projection so as to be removed. For this reason, an unnecessary hole is not made.
However, since the spot-welded portion is brought into a kind of a quenched state at the time of deposition, this portion is harder than the other portion of the panel, and a boring machine such as a drilling machine cannot be used for a spot-welding removal work and thus a hand drill is used. For this reason, it is necessary to give a so-called thinning process for making a chisel width to be small so as to form a cutting edge at the central portion of the drill is given to a drill to be used for the spot-welding removal work.
Conventionally, this thinning process has been performed manually by using a bench abrasive machine, but since a very skilled technique has been required for the thinning process, it was difficult to perform the thinning process accurately for a short time.
From this point of view, the inventor of this application has suggested a drill abrasive apparatus having a thinning mechanism in Japanese Utility Model Application Laid-Open No. 7-15247 (1995).
This drill abrasive apparatus was extremely excellent in that the thinning process can be performed by a simple operation without requiring a skilled technique, but this abrasive apparatus could not form a thinning shape (Z-type thinning) which is the most suitable for the above-mentioned spot-welding removal because of problems of a whetstone shape and a positional relationship between the drill and the whetstone and the like.
The present invention is devised in order to solve the above problems, and its object is to provide a drill abrasive apparatus which is capable of being used suitably for removal of spot-welding to be used for a body panel or the like of a vehicle and of grinding a candle-shaped drill requiring Z-type thinning.